Proper navigation of an aerial vehicle is based on the ability to determine a position of the aerial vehicle. Some navigation systems display for a pilot an onboard map database keyed to the current position of the aerial vehicle. Typically, a Global Positioning System (GPS) receiver and an Inertial Measurement Unit (IMU) are used to determine the aerial vehicle's position. However, GPS requires radio frequency (RF) signal reception from satellites that can be interfered with and are not always available. When GPS is not available, digital map data, which is typically used for terrain obstacle avoidance, is unusable because there is no position reference available from GPS to use to provide an accurate position solution and orient a track of movements of the aerial vehicle onto the map.
Another significant problem for aerial vehicles is avoidance of terrain obstacles when flying near the ground. Cables represent a particularly insidious hazard, as they are difficult to see even during daylight flight in good visibility conditions. Collisions with cables and terrain obstacles result in dozens of injuries and deaths annually, with the problem being more severe in the armed services where it is common to fly in unsurveyed areas and for low level flights in remote areas. Even in surveyed areas, digital map data has insufficient resolution to symbolically represent or display cables. The survey is often days or weeks out of date and thereby does not contain current information on artificial obstacles that move into an area after the survey is completed. Currently, some aerial vehicles require the use of multi-mode radar (MMR) to operate in low to zero visibility flight environments. However, MMR has a large radar signature that increases the potential of detection by unfriendly forces.